Is there any difference other than 13mm difference between the 12mm plenum spacer and the 25mm plenum spacer? What would be more suitable for a 3.8L VS Ute?

The difference between the two plenum spacers is that the thicker one offers greater air distribution between cylinders, however, this is at the expense of throttle response. This isn't a problem when running the thicker of the two plenum spacers with boost (which is what we recommend), however, if you install this on a naturally aspirated ecotec then throttle response will suffer. Running the 12mm plenum spacer on a naturally aspirated ecotec on the other hand, works well. In saying the above though, unless you're already running a manifold insulator, I would recommend this item first over the plenum spacer.